1. Field of the Invention
This invention relates to methods of fabrication and particularly to a new and improved method of fabricating a hollow article.
2. Description of the Prior Art
Certain articles, such as rotor blades in a gas turbine engine, are often fabricated to be hollow, that is, to have internal cavities, in order to reduce weight and promote cooling. One method of fabricating such hollow rotor blades is by casting, using a ceramic mold to form the external surface and a ceramic insert, or core, to form the internal surface. Molten metal is poured between the mold and core and the assembly is cooled until the metal solidifies. The mold is then removed and the core dissolved by chemical means, leaving the finished blade. Although this method of fabrication has been successfully employed with many materials at various temperatures, the method has certain limitations. For example, in the fabrication of certain blades, such as eutectic blades, it has been found that during solidification of the cast metal alloy, carbides form on the surfaces of the newly formed blade as a result of a reaction between the ceramic mold, the core and the cast material. Chemical removal of the carbides can leave sharp cornered pits in the internal and external surfaces of the blade. Such pits can result in reduction in blade life. To avoid such pits, the carbides are thus removed from the surfaces of the blade mechanically. This extra step, however, increases fabrication time and cost.
Another method used for fabricating hollow blades has been by casting the blade as two separate halves, separately machining or milling cavities in the inner portions of each half, and then fastening or bonding the halves together. Although this method of fabrication has also been successfully employed, it has some drawbacks. For instance, casting the blade halves separately can result in a mismatch between the halves when they are bonded, requiring additional time and expense for correction. Additionally, the bond line, or interface between the adjacent surfaces of the two halves, normally extends from the leading edge to the trailing edge of the completed blade. The leading and trailing edges of the blade, however, are locations of high stress and high temperature. At such high stress and temperature locations, it is preferable for reasons of structural integrity not to have a bond line.
Further, the bond line often follows the cord line of the blade and is thus curved. When the process for bonding the two halves together requires exertion of pressure against the two halves, it is difficult to exert pressure substantially normal to the bond line, at which angle the pressure will be most effective. Furthermore, application of pressure against the outer surfaces of the halves is difficult since, in the case of a rotor blade, one of the surfaces is concave and the other is convex.
In view of the above-mentioned problems, it is therefore an object of the present invention to provide a method of fabricating a hollow article which reduces or eliminates the formation of carbides on the internal and external surfaces of the article.
Another object of the present invention is to provide a method of fabrication which facilitates proper match-up and fit between parts to be bonded.
Still another object of the present invention is to provide a method of fabrication which improves the structural integrity of the finished article by disposing the interface between adjacent surfaces of parts at locations of low stress.
Yet another object of the present invention is to provide a method of fabrication which improves the bonding between parts of the article by facilitating exertion of pressure on the parts substantially normal to the interface between adjacent surfaces of the parts.